20,349 research outputs found
Early warning signals: The charted and uncharted territories
The realization that complex systems such as ecological communities can
collapse or shift regimes suddenly and without rapid external forcing poses a
serious challenge to our understanding and management of the natural world. The
potential to identify early warning signals that would allow researchers and
managers to predict such events before they happen has therefore been an
invaluable discovery that offers a way forward in spite of such seemingly
unpredictable behavior. Research into early warning signals has demonstrated
that it is possible to define and detect such early warning signals in advance
of a transition in certain contexts. Here we describe the pattern emerging as
research continues to explore just how far we can generalize these results. A
core of examples emerges that shares three properties: the phenomenon of rapid
regime shifts, a pattern of 'critical slowing down' that can be used to detect
the approaching shift, and a mechanism of bifurcation driving the sudden
change. As research has expanded beyond these core examples, it is becoming
clear that not all systems that show regime shifts exhibit critical slowing
down, or vice versa. Even when systems exhibit critical slowing down,
statistical detection is a challenge. We review the literature that explores
these edge cases and highlight the need for (a) new early warning behaviors
that can be used in cases where rapid shifts do not exhibit critical slowing
down, (b) the development of methods to identify which behavior might be an
appropriate signal when encountering a novel system; bearing in mind that a
positive indication for some systems is a negative indication in others, and
(c) statistical methods that can distinguish between signatures of early
warning behaviors and noise
Plasma heating in the very early and decay phases of solar flares
In this paper we analyze the energy budgets of two single-loop solar flares
under the assumption that non-thermal electrons are the only source of plasma
heating during all phases of both events. The flares were observed by the
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Geostationary
Operational Environmental Satellite (GOES) on September 20, 2002 and March 17,
2002, respectively. For both investigated flares we derived the energy fluxes
contained in non-thermal electron beams from the RHESSI observational data
constrained by observed GOES light-curves. We showed that energy delivered by
non-thermal electrons was fully sufficient to fulfil the energy budgets of the
plasma during the pre-heating and impulsive phases of both flares as well as
during the decay phase of one of them. We concluded that in the case of the
investigated flares there was no need to use any additional ad-hoc heating
mechanisms other than heating by non-thermal electrons.Comment: 22 pages, 10 figures, The Astrophysical Journal (accepted, March
2011
Quantitative Analysis of Single-Molecule Force Spectroscopy on Folded Chromatin Fibers
Single-molecule techniques allow for picoNewton manipulation and nanometer accuracy measurements of single chromatin fibers. However, the complexity of the data, the heterogeneity of the composition of individual fibers and the relatively large fluctuations in extension of the fibers complicate a structural interpretation of such force-extension curves. Here we introduce a statistical mechanics model that quantitatively describes the extension of individual fibers in response to force on a per nucleosome basis. Four nucleosome conformations can be distinguished when pulling a chromatin fiber apart. A novel, transient conformation is introduced that coexists with single wrapped nucleosomes between 3 and 7 pN. Comparison of force-extension curves between single nucleosomes and chromatin fibers shows that embedding nucleosomes in a fiber stabilizes the nucleosome by 10 kBT. Chromatin fibers with 20- and 50-bp linker DNA follow a different unfolding pathway. These results have implications for accessibility of DNA in fully folded and partially unwrapped chromatin fibers and are vital for understanding force unfolding experiments on nucleosome arrays
Multi-wavelength Diagnostics of the Precursor and Main phases of an M1.8 Flare on 2011 April 22
We study the temporal, spatial and spectral evolution of the M1.8 flare,
which occurred in NOAA AR 11195 (S17E31) on 22 April 2011, and explore the
underlying physical processes during the precursors and their relation to the
main phase. The study of the source morphology using the composite images in
131 {\deg}A wavelength observed by the SDO/AIA and 6-14 keV revealed a
multiloop system that destabilized systematically during the precursor and main
phases. In contrast, HXR emission (20-50 keV) was absent during the precursor
phase, appearing only from the onset of the impulsive phase in the form of
foot-points of emitting loop/s. This study has also revealed the heated
loop-top prior to the loop emission, although no accompanying foot-point
sources were observed during the precursor phase. We estimate the flare plasma
parameters viz. T, EM, power-law index, and photon turn-over energy by forward
fitting RHESSI spectral observations. The energy released in the precursor
phase was thermal and constituted ~1 per cent of the total energy released
during the flare. The study of morphological evolution of the filament in
conjunction with synthesized T and EM maps has been carried out which reveals
(a) Partial filament eruption prior to the onset of the precursor emission, (b)
Heated dense plasma over the polarity inversion line and in the vicinity of the
slowly rising filament during the precursor phase. Based on the implications
from multi-wavelength observations, we propose a scheme to unify the energy
release during the precursor and main phase emissions in which, the precursor
phase emission has been originated via conduction front formed due to the
partial filament eruption. Next, the heated leftover S-shaped filament has
undergone slow rise and heating due to magnetic reconnection and finally
erupted to produce emission during the impulsive and gradual phases.Comment: 16 Pages, 11 Figures, Accepted for Publication in MNRAS Main Journa
Plasmon reflections by topological electronic boundaries in bilayer graphene
Domain walls separating regions of AB and BA interlayer stacking in bilayer
graphene have attracted attention as novel examples of structural solitons,
topological electronic boundaries, and nanoscale plasmonic scatterers. We show
that strong coupling of domain walls to surface plasmons observed in infrared
nanoimaging experiments is due to topological chiral modes confined to the
walls. The optical transitions among these chiral modes and the band continua
enhance the local ac conductivity, which leads to plasmon reflection by the
domain walls. The imaging reveals two kinds of plasmonic standing-wave
interference patterns, which we attribute to shear and tensile domain walls. We
compute the electronic structure of both wall varieties and show that the
tensile wall contain additional confined bands which produce a
structure-specific contrast of the local conductivity. The calculated plasmonic
interference profiles are in quantitative agreement with our experiments.Comment: 14 pages, 5 figure
HST/FOS Eclipse Observations of the Nova-like Cataclysmic Variable UX Ursae Majoris
[abridged abstract]
We present and analyze Hubble Space Telescope observations of the eclipsing
nova-like cataclysmic variable UX UMa obtained with the Faint Object
Spectrograph. Two eclipses each were observed with the G160L grating (covering
the ultraviolet waveband) in August of 1994 and with the PRISM (covering the
near-ultraviolet to near-infrared) in November of the same year. The system was
50% brighter in November than in August, which, if due to a change in the
accretion rate, indicates a fairly substantial increase in Mdot_acc by >~ 50%.
Model disk spectra constructed as ensembles of stellar atmospheres provide
poor descriptions of the observed post-eclipse spectra, despite the fact that
UX UMa's light should be dominated by the disk at this time. Suitably scaled
single temperature model stellar atmospheres with T_eff = 12,500-14,500 K
actually provide a better match to both the ultraviolet and optical
post-eclipse spectra. Evidently, great care must be taken in attempts to derive
accretion rates from comparisons of disk models to observations.
One way to reconcile disk models with the observed post-eclipse spectra is to
postulate the presence of a significant amount of optically thin material in
the system. Such an optically thin component might be associated with the
transition region (``chromosphere'') between the disk photosphere and the fast
wind from the system, whose presence has been suggested by Knigge & Drew
(1997).Comment: 35 pages, including 12 figures; to appear in the ApJ (Vol. 499
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